In the television industry there is an ongoing desire to combine video signals in a variety of ways to create various special effects For example, a common function of a digital video effects system is to compress a full field of video to a size smaller than the raster area occupied by a full size field of television signal. The smaller raster is then displayed at a predetermined location within the larger raster of video
Framestores recently have been developed for use in the video industry to store video information for the creation of special effects Such devices typically include a memory device capable of holding at least one complete raster of video information, i.e., either a field or a complete frame of video information. One such framestore system is described in U.S. Patent Application entitled "A Recirculating Special Effects Video Framestore" by David E. Trytko, filed Sept. 13, 1989 as U.S. patent application Ser. No. 07/407,722, which was a continuation of copending application Ser. No. 07/195,370 filed May 11, 1988, now abandoned, which was a continuation of copending application Ser. No. 06/943,282 filed Dec. 8, 1986, now abandoned, which was a continuation-in-part of copending application International Application No. PCT/US86/00733 filed Apr. 11, 1986, which was a continuation-in-part of copending application Ser. No. 06/722,532 filed Apr. 12, 1985, now abandoned and assigned to the same assignee as the present patent application the details of which are hereby incorporated by reference.
In creating special effects, digital video signals are typically combined through the use of key signals. Two unprocessed video signals cannot be directly combined because the combined signal gain would be the sum of the initial gains of each unprocessed video signal and could exceed the maximum gain resource of the system, i.e., the combined video signal can have a maximum gain no larger than the maximum gain of any of the component video signals. Thus, if all component video signals in the system are allowed to vary in gain anywhere from 0 to 1 volt, the maximum gain excursion of a composite video signal consisting of a combination of two or more component video signals can be no more than from 0 to 1 volt The key signals are used according to the teachings of the invention to cut the gain of component video signals according to calculations relating to the transparency factor of each signal, the maximum available gain for the composite signal and the priority of the various signals. By using separate key signals for each video signal, the key signals can be independently processed to control the gains of the component video signals in such a manner that the final composite video signal uses all the available gain resource without the injection of spurious noise. That is, prior to combining video signals, each individual video signal is gain adjusted by its respective key signal in such a manner that the composite video signal does not exceed an overall reference gain which is the maximum allowable gain resource available to any video signal in the system. A more complete understanding of key signals in video signal processing can be obtained from U.S Patent Application entitled "Apparatus and Method for Processing Video Signals With Key Signals in Accordance With Previous Processing of the Video Signals " by John F. Bloomfield, filed Sept. 1, 1989 as U.S. patent application Ser. No. 07/403,898, which was a continuation of copending application Ser. No. 07/191,778 filed May 2, 1988, now abandoned, which was a continuation of copending application Ser. No. 07/119,223 filed Nov. 3, 1987, now abandoned, which was a continuation-in-part of copending application Ser. No. 06/851,195 filed Apr. 14, 1986, now abandoned, which was a continuation-in-part of copending application International Application No. PCT/US86/00734 filed Apr. 11, 1986, which was a continuation-in-part of copending application U.S. Ser. No. 06/722,532 filed Apr. 12, 1985, now abandoned, and also assigned to the same assignee as the present patent application the details of which are hereby incorporated by reference.
Another well known device used in video effects systems is a combiner which is a device to do a "combine" of two or more video signals. A "combine" is effected by combining two or more channels of video information in such a way as to make the scenes or images appear in a desired relationship with each other, e.g., one in front of the other, a transparent image, one image flying over another, one image moving and leaving a trail, etc. One such combiner is illustrated, by way of example in U.S. Pat. No. 4,758,892 entitled "Circuit for Producing a Video Combine from Multiple Video Images" filed on Apr. 27, 1984 by John F. Bloomfield, assigned to the same assignee as the present patent application the details of which are hereby incorporated by reference.
In the video effect system described in the above identified U.S. patent application Ser. No. 407,722, video information is held in a memory sometimes hereafter referred to as a framestore or store and is recirculated. This recirculated video is combined with new or input video information in a combiner during recirculation. The recirculated video is processed or "cut" by a key signal in a controllable manner during each recirculation to reduce the gain of the signal. This cut allows combining the recirculated video signal with other video having a known gain to produce desired video effects in the form of an output video signal having a gain which does not exceed the system's maximum allowable gain. It is a limitation of such systems that once video information in the store has been processed or cut by the key signal during recirculation, the lost gain cannot be restored since to increase the gain of a video signal by multiplying it by a value greater than 1 only adds noise since there is no new video information in the multiplication operation. Thus, the cut video information is recirculated back into the frame store and overwrites the video information stored therein. The video information that existed before the cut is therefore forever lost. This is disadvantageous since it is desirable for certain special video effects to be able to make the recirculated video information from the framestore appear at various gain levels, disappear and then reappear at any gain level compatible with the maximum allowable system gain.